1. Field of the Invention
The present invention relates to a laminated film and, more particularly, to a laminated film suitable for use in a touch panel which is extensively used for electronic devices, household electrical utensils and so on.
2. Description of Related Art
Recently, due to its excellent electrical characteristics as inherent in piezoelectric inorganic material and its remarkable processability as inherent in readily processable polymer, there have increasingly been used a sort of flat keyboard switches, as called touch panel, for household electrical utensils such as television sets, audio devices and microwave ovens, electronic equipment and devices such as desk top calculators, copying machines and facsimile machines, devices and equipment for use in a variety of experiments, production and processing, and so on.
The touch panel basically comprises two sheets of transparent conductive films, each having a conductive layer made from an indium/tin oxide thin film as an electrode, which are disposed so as to allow their electrodes to face each other through a spacer. The touch panel is attached to a surface of a display member such as LCD, CRT display, plasma display, EL display or the like. As the desired electrode is pressed, electricity is passed through the two electrodes to transmit a given signal to a driving unit, thereby operating the display unit. Although a transparent and extremely flexible film such as a polyester film is used as a base film for the transparent conductive film, the film may present the problems of appearance because of the likelihood of its surface to be damaged and to appear dirty and because of quickly turning letters or characters under the film illegible. Furthermore, the film may be torn off or damaged during use for a long time period, thereby giving rise to circuit accidents or troubles of accessory devices.
In order to improve hardness of the surface of the touch panel, attempts have been made to coat the surface of the base film with a resin curable with ultraviolet rays, as disclosed in Japanese Patent Unexamined Publication No. 115,613/1987. This hard coating compound however, cannot provide a coat having a sufficient degree of adhesion when it is coated by a one-time coating.
Hard coating compound of silicone type, as recently developed, provides an excellent degree of adhesion yet requires to be coated twice or to be cured by applying heat, whereby they have the drawbacks that it takes a long period of time or it requires laborious work for processing and investment and processing costs are rendered expensive. Further, these coating compounds present the disadvantages that the thin film consisting of a thin film substrate may be curled when they are coated on the film, due to their large shrinkage upon curing and that a coating film may be cracked when they are coated in a thick layer.
On top of that, there is known a touch panel of the type to be used for desk top electronic calculators, which differs from those conventional touch panels to be attached to the surface of the display member in terms of unnecessity to use a transparent base film. The touch panel of this kind also has substantially the same problems and difficulty as the other conventional touch panels have.